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By Nora Schultz (Image: Martin Plath) Mating fish don’t like an audience, it seems. When another male spies on them they change their mind about which female they prefer. The findings may alter the way we think about mate choice driving evolution, researchers say. Male molly fish of the species Poecilia Mexicana normally prefer to mate with large females who produce more eggs. In mate choice experiments, a male will spend 80% of its time near large females and only 20% near smaller ones. But when Martin Plath at the University of Potsdam in Germany and colleagues stuck a glass container holding another male into the tank to let him watch the show, the first molly changed his mind. Under the gaze of the intruder, he began to pay equal attention to both large and small females. Being watched by a green swordtail (Xiphophorus hellerii), on the other hand, did not faze the mollies at all – they only slightly reduced their preference for large females. “We think that the molly does this to avoid sperm competition,” Plath told New Scientist. “It’s likely that the other male will share the preference for large females, so it makes sense for the molly to not invest all his sperm into one female.” Another possible explanation would be that the desire to fight a competitor distracts the very aggressive mollies from their true mate preference, although Plath considers this less likely. “We have just repeated the experiment with a completely non-aggressive group of fish and got the same results,” he says. Peter McGregor at Cornwall College in the UK, who studies communication networks in animals, says the results highlight how sensitive mate choice can be to even subtle interactions between individuals. “The other male cannot get actively involved and still his passive presence has this dramatic effect”, he says. Previous studies have shown that female guppies – and humans – prefer males that are popular with other females, and that human males adjust their sperm production to perceived competition, but this is the first direct evidence that mate choice can be reversed by an observer. “Mate preferences are often seen as drivers of evolutionary change,” says Plath. “These results show how important it is to consider them in the appropriate social context.” Journal reference: Animal Behaviour (DOI: